This invention relates to water-based hydraulic and metalworking fluids, in particular those fluids which are thickened with a substantially water-swellable copolymeric thickening agent.
Petroleum oils have traditionally been used as hydraulic fluids. Such oils exhibit good Newtonian viscosity behavior. A Newtonian fluid is a fluid that possesses a viscosity which is independent of the velocity gradient. Thus, the shear stress (.tau.) is related to the shear rate (.gamma.) by the equation: EQU .tau.=.eta..gamma.
wherein .eta. is the shear rate independent viscosity. Further, petroleum oils have a viscosity that is fairly constant throughout the lifetime of the fluid at prolonged high shear rates. This mechanical stability to shear degradation is a desired property of hydraulic fluids. The shear stable Newtonian viscosity of a typical hydraulic oil is generally in the range of 10 to 100 centistokes at 100.degree. F.
Water-based lubricant products are gaining popularity due to shortages of petroleum base supplies, environmental concerns caused by problems in disposing of oil-based wastes, cost incentives and fire safety considerations. Typically, a water-based hydraulic fluid consists of several water-soluble or emulsifiable additives such as corrosion inhibitors (alkanolamines), lubricity aids (long chain carboxylic acid salts) and/or extreme pressure additives (zinc dialkyldithiophosphates, phosphate esters, borates, etc.). However, such an additive package has a viscosity that is essentially equal to that of water. It is desirable to thicken such a water-based lubricant with a substantially water-swellable thickening agent to overcome the problems associated with the use of a low viscosity fluid.
Increased viscosity of the water-based hydraulic fluids is desirable for several reasons. In particular, thickened fluid can aid in the operation of system valves which have been designed to work specifically with oil-based fluids. Further, thickened fluids are less prone to experience leaking though small holes or cracks in the hydraulic system. Higher pump efficiencies are obtainable with thickened fluids, especially at high loads, and such fluids exhibit wear prevention characteristics in both hydrodynamic and elastohydrodynamic wear modes. It is desirable to provide a viscosity which is relatively constant throughout the lifetime of the fluid and relatively constant at varying shear rates. Shear rates in hydraulic vane pumps are estimated to be as high as one million reciprocal seconds.
For water-based hydraulic fluids, a polymer solution having a mechanically stable viscosity of about 10 to about 100 centistokes at 100.degree. F. and a viscosity independent of shear rate at shear rates approaching up to about 10.sup.6 sec.sup.-1 is desirable. One way of describing the viscosity dependence on shear rate is through the use of the Power Law: EQU ln .tau.=N ln .gamma.+ln K.
Here, the shear stress (.tau.) is found to vary in a nonlinear manner with shear rate (.gamma.). Thus, the viscosity changes with changes in shear rate. N is a measure of the extent of deviations from Newtonian behavior. A Power Law N value of 1.0 indicates a Newtonian fluid. Anything less than 1.0 is said to be shear-thinning. The K value relates to the fluid viscosity at a shear rate of 1 sec.sup.-1. Further, for the sake of economic efficiency, it is desirable to keep the polymer concentration as low as possible. However, it is not always possible to provide a polymer system that exhibits a desired, mechanically stable hydrodynamic size and the desired Newtonian viscosity while maintaining a high polymer thickening efficiency.
Water-soluble polymers can be made in a variety of physical structures and molecular weights. High molecular weight linear polymers are highly efficient thickeners. However, such polymers exhibit non-Newtonian viscosity behavior and suffer from mechanical degradation at high shear rates. Reduction in molecular weight of the linear polymers increases the Newtonian character and mechanical stability of the thickener. Unfortunately, such low molecular weight polymers require high concentrations to thicken the fluid and thus are not economical.
It is desirable to produce compositions which, at low concentrations, exhibit a substantial thickening effect on the water in the aqueous hydraulic systems formed thereby, and provide the aqueous system with high viscosity and enhanced shear stability. It is also desirable that the viscosities in the aqueous hydraulic fluid systems employing the thickeners approach the viscosities of oil-based hydraulic systems, i.e., about 10 to about 100 centistokes at 100.degree. F.